The long term objective of our current research is to understand the mechanisms by which cells move within a three-dimensional, multicellular mass. This movement is more complex than migration of an isolated cell on a flat substrate, because cells within a mass must gain traction on each other and also clear paths for themselves within and through the cell mass. We study such "3D cell motion" using time-lapse 3D microscopy which permits us to observe locomotion in living specimens over extended time periods. Current research focuses on the role of the myosin II molecule in cell motility within the multicellular mass of the model organism Dictyostelium. Using a GFP-tagged mysoin II, we have shown that retraction of the cell rear is accompanied by a characteristic redistribution of the myosin at the cell rear. We have also shown that that this redistribution of myosin is abberant when certain myosin mutants are examined, and that rear retraction is also concomitantly affected. These results are the first to demonstrate a specific functional role for myosin II in rear retraction of a cell.